One arrangement for distributing large data files to multiple locations utilizes broadcasting of the data. Typically broadcasting utilizes so-called "single-point to multi-point" communications broadcast technology. For some applications, transferring data in this manner is efficient and cost effective. This technique is presently used with satellite technology to broadcast television signals and the like. One limitation of wireless broadcast technology is that, transmission path errors may result in each recipient of the broadcast data receiving different data because of the different transmission paths between the broadcast point and the multiple receiving points. Furthermore, because different transmission paths may produce different reception errors, all recipients of the broadcast may not incur the same error.
This limitation excludes use of conventional single-point to multi-point broadcast technology for certain applications for which data files must be received without error at multiple locations. For example, communication of certain financially oriented data or software must be delivered without error. To transfer data with high accuracy requires that information be exchanged between the transmitting point of the data and each recipient of the data. The information exchanged may be as simple as an acknowledgment that the data has been received error free, or may include requests for retransmission of part or all of the data files. Typically the transfer of data requiring high accuracy in reception is accomplished using "point-to-point" communications. Use of a point-to-point communication system to deliver large amounts of the same data to a large number of recipients is generally not time or cost effective.
Communication satellites are used to transmit and receive large amounts of data over high bandwidth communication links. To achieve the high bandwidths, the frequencies utilized for satellite communications links are in the ultra high frequency (uhf) and above radio spectrum. Although the use of UHF and higher frequencies permits the transfer of information at high data rates, signals transmitted at such high frequencies are received only at receivers which are in line of sight with the satellite.
Communication satellites are characterized as being high earth orbit (HEO) or low earth orbit (LEO) satellites. HEO satellites include Geostationary Earth Orbiting (GEO) satellites which are in geosynchronous earth orbit. GEO satellites are particularly well suited for the broadcast of information because the height above earth assures that the radiation pattern provides for line of sight communications over up to a hemisphere of earth. However, because of the GEO satellite distance from the earth, on the order of 22,000 miles, propagation times between an earth based station transmitting data and a second earth based receiver become significant. Round trip propagation delay between two earth stations can be approximately one half second. Because of the significant propagation delay, returns signals from a receiving station indicating errors in received data are not received until an additional one half second of data has been transmitted. Furthermore, the range distance between the receiving station and a GEO satellite requires relatively high power to be transmitted if a return signal is to be transmitted through a GEO. This is incompatible with low cost terminals. Because of the hemispheric coverage of GEO satellites, they are particularly well adapted to broadcast of data to large numbers of recipients. However, there is no present provision for the re-broadcasting of data for correction of reception errors in data broadcast from HEO or GEO satellites.
In contrast, LEO satellites because of their proximity to earth do not have the large propagation delays in round trip communications between two earth stations served through one or more LEO satellites. However, because of the proximity of LEO satellites to the earth's surface, the line of sight coverage area is relatively small. Therefore, LEO satellites generally are not as effective as HEO satellites in widespread broadcasting of data but serve well for point-to-point communication. Typically, to provide communication coverage of the entirety of the earth, both LEO and GEO satellites are provided in constellations of multiple satellites. One known LEO satellite constellation arrangement has been well documented and is known as the IRIDIUM.RTM. satellite system manufactrured by Motorola, Inc. GEO satellite arrangements are likewise known.
It is desirable to provide an arrangement which provides the data broadcast capabilities of a single point to multiple point arrangement along with the advantages of point to point communication systems to allow more efficient, less costly, and more reliable broadcast delivery of data.